1. Field of the Invention
This invention relates to video recording and/or reproducing apparatus, and is more particularly directed to video tape apparatus whereby editing of a video signal may be carried out.
2. Brief Description of the Prior Art
Conventional video tape recorders (VTRs), of the type having one or more rotating heads which scan the video tape at an angle to the direction of advancement thereof, can be adapted to be set into editing modes so that a new scene is recorded continuously after a previously recorded scene. In one such mode, sometimes called an assemble mode, after one scene is recorded, the VTR is stopped, following which another scene is recorded immediately thereafter. In another mode, sometimes called an insert mode, a new scene is recorded between two scenes which are recorded previously on the tape.
In either the insert or assemble mode, the edit points, i.e., the points connecting the respective successive scenes, are determined, for example, by an operator viewing the video picture on a monitor. In a usual edit operation, the operator stops the VTR at a selected point by depressing a pause button. Then, when the operator is ready to record the new video material, the VTR is released from its pause mode and is set into its record mode, and a new video scene is recorded beginning at the edit point.
Whenever such assemble or insert recording operations are performed, care must be taken to prevent the video signal from generating confusion or noise at the edit point. For this reason, at the edit point the pitch of the recording head traces should correspond to the pitch of the tracks previously recorded on the tape. If the pitch of the recorded tracks changes discontinuously at the edit point, the reproduction of the recorded signals in the tracks around the edit point will become degraded. More particularly, when the track pitch changes, the reproducing heads will not accurately scan the recorded signal, and the reproduced picture will become noisy. Also, when the track pitch changes, the reproduced vertical and horizontal synchronizing signals will occur at a time that does not correspond to previously picked up synchronizing signals, and synchronization of the reproduced video will become unreliable.
In order to avoid changes in the track pitch at an edit point, conventional techniques take advantage of a control track recorded at one edge of the video tape, and recognize that the positions of the video tracks correspond accurately to positions of the control signal recorded on the tape. That is, in the recorded signal after editing, the pulses of the control signal have a constant pitch .tau..sub.c.
In order to avoid changes in track pitch, the recording of the tracks of the new video signal should be controlled by the tracking servo system of the VTR, so that the tracks are recorded at locations determined by the recorded control signal corresponding to a previously-recorded scene, while the recording heads are rotated in synchronism with the vertical synchronizing signal contained in the new video signal.
To carry out recording under the control of the tracking servo system, the tape must be reversed for at least a short distance ahead of the edit point. Then, the new scene is recorded, beginning at the edit point, only after the tape has been advanced for that short distance. Thus, the track pitch for the new video signal matches that of the previously recorded video signal, and picture degradation or synchronization confusion is avoided at the edit point.
According to conventional editing techniques, the tape is returned mechanically for a short distance to reverse it for the above-mentioned short distance. In a typical editing technique, the VTR is stopped at or near the intended edit point, and a lever is manually actuated to energize a solenoid plunger. The plunger reversely rotates a guide roller of the VTR, and thereby loosens the tape an amount approximately 3.5 times as long as the pitch of the recorded control signal. The amount of tape so loosened is then taken up on a tension regulator to reverse the tape, relative to the heads, by the amount 3.5 times the pitch of the control signal. A timer is preset by the running time corresponding to this amount, and the timer starts the recording of the new scene the preset time after the tape is started in the forward direction. That is, the recording of the new scene is commenced automatically, under the control of the preset timer, as the tape passes the edit point.
With the conventional editing technique, as the returned amount of tape is arrived at mechanically, the tape advancement mechanism must be stringently adjusted and the preset timer must be controlled with extreme accuracy to ensure that the new video signal is commenced precisely at the edit point, and so that the rotary recording head will be accurately synchronized with the previously recorded video signals. Nevertheless, because the conventional operation is carried out mechanically, even if the timer and tape advancement mechanism are adjusted as precisely as possible, the returned amount of tape will not always be constant because of factors such as tape stretch and temperature change. Thus, the conventional technique cannot ensure that the new scene will commence precisely at a predetermined edit point.
Alternatively, it has been proposed to arrange a VTR to begin recording the new scene immediately after three occurrences of the control signal have been counted, where the tape is mechanically returned by an amount of 3.5 times the pitch of the control signal. However, in such proposed technique, because the edit point of the tape is not in any fixed relation to the control signal, there is a possibility that the tape can be stopped at the vicinity of a recorded occurrence of the control signal. If this should occur, mere vibration of the stopped video tape can cause the control head, which picks up the control signal, to misinterpret a single occurrence thereof as a multiple occurrence. Thus, when the advancement of the tape is commenced immediately prior to the recording of the new scene, a miscount of the control signal may result in the new scene being recorded at the wrong place on the video tape, and, perhaps also, before the phase of the rotary head is properly adjusted.
Furthermore, when the video tape is stopped in advance of the edit point to await the recording of the new signal, the angle of the recording/reproducing head trace relative to the tape is different from the angle thereof when the video tape is being advanced at normal speed. As a result, the recorded tracks on the tape will not be scanned precisely. A noisy reproduced video signal can result unless the trace of the recording/reproducing head is caused to intersect approximately the middle of the recorded track at which the tape is stopped. Unfortunately, no provision has previously been incorporated in an editing system for a VTR to ensure a substantially noise-free signal is reproduced when the VTR is stopped during an editing operation.
Completely electronic video tape editing systems have been proposed to deal with one or more of the above-mentioned problems. However, any previously proposed all-electronic systems have been extremely complex and hence unduly expensive.